CD40 is a tumor necrosis factor (TNF) receptor family member that plays an important role in B cell development, lymphocyte activation, and antigen presenting cells (APC) function. CD40 expression on epithelium, leukocytes, and vascular endothelium is elevated in organ-specific autoimmune diseases as well as systemic autoimmunity such as systemic lupus erythematosus (SLE). Disruption of the CD40L/CD40 signaling pathway reduces production of proinflammatory cytokines such as IL-23 and TNF, reduces T helper cell differentiation and function, and inhibits macrophage activation in patients with chronic inflammatory diseases such as Crohn's disease. The interaction of CD40 with CD40L induces both humoral and cell-mediated immune responses. CD40 regulates this ligand-receptor pair to activate B cells and other antigen-presenting cells (APC) including dendritic cells (DCs).
CD40 is a 48 kDa type I transmembrane protein (van Kooten, J Leukoc Biol. 2000 January; 67(1):2-17) that is expressed on a wide range of hematopoietic (lymphocytes, monocytes, dendritic cells) and non-hematopoietic (epithelium, endothelium, fibroblasts) cell types. CD40L is expressed primarily on activated T cells, B cells, and platelets. Much of the understanding of CD40/CD40L biology comes from the interaction between APCs (CD40 expression on either dendritic cells (DC) or B cells) and CD40L-expressing T cells. On resting B cells, CD40L engagement drives B cell activation, proliferation, and memory B cell development (Kehry, Immunol. 1996 Apr. 1; 156(7):2345-8). CD40 signaling is also required for immunoglobulin class switching and germinal center formation. The importance of the CD40/CD40L signaling pathway in B cell biology is evident in CD40- or CD40L-deficient mice which lack germinal centers and T-dependent antibody responses are suppressed. However, T-independent IgG responses remain intact in CD40−/− mice suggesting that it is cell-cell interaction that is lacking in these mice. CD40-deficient mice also have deficits in the T cell compartment. Signaling through CD40 on dendritic cells upregulates MHC class II as well as various costimulatory molecules such as CD80 and CD86 and promotes maturation of DC. Mature DC stimulate activation and survival of CD4+ T cells through production of cytokines such as IL-2 and IL-12. Inefficient T cell priming appears to be the primary cause of compromised T-dependent humoral responses in CD40L−/− mice (Grewal, Nature. 1995 Dec. 7; 378(6557):617-20). A similar B cell phenotype can be seen in humans with X-linked hyper IgM syndrome. These patients suffer from primary immunodeficiency due to mutations in the CD40L locus that abrogates CD40/CD40L signaling. These individuals have elevated IgM levels and cannot produce IgA, IgG, and IgE resulting in an increased risk of opportunistic infections (Adriana, J Clin Immunol. 2008 May; 28 Suppl 1:S62-6).
CD40 signaling pathway is central to the conversion of resting or naïve lymphocytes and APCs to an activated/mature phenotype. Although T cell priming and B cell activation can occur in the absence of CD40/CD40L signaling, this pathway is required for generating a robust adaptive immune response. Engagement of CD40 by CD40L results in the recruitment of TNF receptor associated factors (TRAFs) to the cytoplasmic domain of CD40 (Bishop, Adv Exp Med Biol. 2007; 597:131-51). Phosphorylation of various TRAF proteins results in activation of both canonical and non-canonical NFkB pathways. In addition, JAK3 association with CD40 cytoplasmic tail results in STATS activation which induces maturation of DC as well as TNF and IFNγ production. TRAF6-dependent PI3K activation is a critical survival signal in DC while TRAF2/TRAF6 have redundant functions in NFkB activation and upregulation of CD80 expression (Hostager, J Biol Chem. 2003 Nov. 14; 278(46):45382-90). TRAFs 2, 3, 5, and 6 have all been shown to play an important role in immunoglobulin class switching mediated by CD40 signaling (Leo, Proc Natl Acad Sci USA. 1999 Feb. 16; 96(4): 1421-1426).
CD40/CD40L signaling pathway has been implicated in the pathogenesis of many autoimmune diseases including systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), multiple sclerosis, rheumatoid arthritis, and Sjogren's syndrome (Law and Grewal, Adv Exp Med Biol. 2009; 647:8-36). CD40 expression is elevated on macrophages, endothelium, epithelium, and B cells in tissues damaged by chronic autoimmunity including kidney, intestine, and joints (Borcherding, Am J Pathol. 2010 April; 176(4):1816-27; Sawada-Hase, Am J Gastroenterol. 2000 June; 95(6):1516-23). Soluble CD40L is elevated in patients suffering from SLE, IBD, and Sjogren's syndrome consistent with inflammatory burden in these patients.
Some of the earliest evidence the CD40/CD40L pathway in chronic intestinal inflammation came from preclinical models where anti-CD40L mAbs protected rodents from experimental colitis (de Jong, Gastroenterology. 2000 September; 119(3):715-23; Liu, J Immunol. 2000 Jun. 1; 164(11):6005-14; Stuber, J Exp Med 1996 Feb. 1, 183(2):693-8). Reduction in disease activity scores were associated with reduced pro-inflammatory cytokine production in the gut and protection from chronic body weight loss. Similar results were observed in animals that were genetically deficient for CD40 or CD40L (de Jong, Gastroenterology. 2000 September; 119(3):715-23). Treatment of mice with anti-CD40L mAbs after disease onset is still effective in reducing disease activity suggesting that this pathway is critical for maintenance of chronic inflammatory disease. In addition, CD40 agonist antibodies are sufficient to drive intestinal inflammation in mice that lack lymphocytes (Uhlig, Immunity 2006 August; 25(2):309-18). More recent data using CD40 siRNA also point to an important role for CD40 signaling in colitis (Arranz, J Control Release. 2013 Feb. 10; 165(3):163-72). In Crohn's disease, lamina propria monocytes and epithelium express high levels of CD40 and CD40+ monocytes are enriched in peripheral blood. Furthermore, polymorphisms in the CD40 locus have been linked to increased susceptibility to IBD. In Crohn's patients treated with anti-TNF antibodies, transcriptional profiling indicates that CD40 mRNA levels decrease in patients with an adequate drug treatment response. However, in patients with a poor response to TNF inhibitors, CD40 mRNA levels are unchanged suggesting that CD40-dependent, TNF-independent pathways may promote inflammation in these patients. Studies suggest that inhibition of CD40 mediated signaling is important in the pathogenesis of IBD as well as other autoimmune diseases. Accordingly, there remains a need for antagonist anti-CD40 antibodies, and antigen-binding portions thereof, that can be used for therapeutic purposes for treating chronic inflammatory diseases and disorders, such Crohn's disease.